LPCVD apparatus is widely used in semiconductor thin film preparation, for example, LPCVD apparatus is used for deposition of polycrystalline thin-film layer in DMOS preparation.
FIG. 1 illustrates basic structure of LPCVD apparatus of the prior art. As shown in FIG. 1, LPCVD 10 includes a reaction furnace, which is often referred to as furnace tube. The wafers to be deposited are placed in the reaction furnace, and reaction gas is ventilated before semiconductor thin films of various properties are generated under specific process parameters. Conventionally, the reaction furnace includes furnace opening part 110, furnace tail part 130 and furnace body part 120 therebetween. The wafers are sent in the reaction furnace through the opening of the furnace opening part 110. Specially, wafer Cassette 900 is used to carry wafers 910 to easily go into or out of the reaction furnace. As shown in FIG. 1, in the thin film growth process, the wafer cassette 900 carrying the wafers 910 is placed between the furnace opening part 110 and the furnace tail part 130, i.e. being placed at the furnace body part. Meanwhile, a port is arranged at the furnace tail part 130 to connect a vacuum pump to vacuum the reaction furnace after the wafers 910 are placed into the reaction furnace but before the reaction gas is ventilated. At the furnace opening part 110 is arranged a reaction gas input pipeline 140, on which can be arranged with flowmeter 141 and pneumatic valve 142. During the semiconductor thin film growth, the reaction gas, for example, SiH4 gas for preparation of polycrystalline silicon, is continuously ventilated into the reaction furnace through the reaction gas input pipeline 140.
However, when the LPCVD in FIG. 1 is used for semiconductor thin film deposition, there is a disadvantage of uneven characters of the thin films between the wafers, for example, uneven thickness of the thin films between the wafers, uneven grain size of the thin films between the wafers, etc. This is because the distances of multi wafers to the furnace opening part 110 are not equal, and the reaction gas introduced from the furnace opening part 110 is difficult to evenly distribute for each of the wafers, such that the reaction conditions for multi wafers 910 in the wafer cassette 900 are different to some extent, finally resulting in the disadvantage above.
There are two methods to avoid the above disadvantage as much as possible. The first method is to reduce the number of the wafers for the thin film deposition per batch, for example, to reduce the number to 70 pieces per batch, thereby the characteristic difference of the thin films between the wafers in the same batch will be relatively smaller. The second method is to set a temperature difference between the furnace tail part and the furnace opening part, i.e. adjusting temperature distribution of the furnace tube. For example, the temperature at furnace tail is adjusted to be 20° C. higher than that of the furnace opening part, thereby reducing the thickness difference of the thin films between the wafers in the same batch.
The above-mentioned first method will greatly limit the efficiency of thin film deposition of LPCVD, usually only 75 pieces per batch. The second method will lead to greater difference of grain size of deposited thin films between the wafers in the same batch; for example, grain sizes of polysilicon thin films are inconsistent.
In view of the above, it is necessary to develop a novel LPCVD apparatus.